The recognition of transparent articles in industry (manufacture of foils, packaging plants, manufacturing of bottles, filling plants, etc.) is a technically demanding task which cannot always be satisfactorily solved by the initially named sensors.
In accordance with the prior art, different principles of solution exists for accomplishing said task:
If the degree of cloudiness of the material to be detected is sufficient, then customary, light barriers with an adapted sensitivity are used. PA1 Furthermore, the polarization characteristics or depolarization characteristics of the transparent articles to be detected can be exploited for their recognition in as much as an attempt is made to evaluate the signal change which occurs as a result of the named characteristics. PA1 The signals delivered by the sensor can be investigated to see whether a pronounced surface reflection and also simultaneously an adequate attenuation can be recognized from which a conclusion can be drawn regarding the presence of a transparent object. PA1 Finally, sensors are known which operate with UV burners and which can detect the transmission changes in the UV range produced by the transparent objects.
In the realization of the above-named known principles, it is necessary to operate with very high system sensitivities as a result of the low attenuation of the materials to be recognized, in order to be able to reliably recognize also materials with a high transmissivity. This necessarily leads to a situation in which the reliability of such systems is impaired by faulty switching, for example, as a result of surface reflections or of disturbing optical radiation.
Since the transparent articles to be recognized frequently have depolarization characteristics which make the evaluation more difficult, the above-named evaluation of the depolarization characteristics cannot always lead to the desired result of the reliable recognition of transparent articles. The evaluation is additionally made more difficult by the fact that the transparent articles have, under certain circumstances, refractive effects (for example lens action with filled bottles) in addition to pronounced surface reflections which are, for example, produced by smooth surfaces, and a low optical attenuation, so that the optical beam flux through the transparent article is not attenuated in the desired manner, but is rather even increased.
A basic problem of the known sensors consequently lies in the fact that they can only reliably recognize quite specific articles which are to be detected.
The sensors which are likewise mentioned above, which operate with UV lamps, make use of the pronounced changes in transmission of the article to be detected in the near UV spectrum. These sensors show, however; as a result of the UV or quartz lamps that are used, a relatively low working life which brings about an increased servicing requirement. The use of the said lamps, moreover, does not correspond to the expected standard of modern industrial sensors because of the required constructional size.
A further substantial problem with the known sensors lies in the fact that the measurement signals that are received are, falsified by disturbing influences. Disturbances are for example, produced by aging or degradation of the components that are used, in particular of the transmitter, by drifting, by contamination, by surface reflection, by lens and mirror effects brought about by the articles to be recognized, and also by temperature influences, in particular, by the temperature dependent plot of the power of the transmitter that is used.